Surfactant protein D binding to alveolar macrophages

Area of Science:

• Pulmonary immunology
• Molecular biology
• Cellular biology

Background:

• Surfactant protein D (SP-D) is a lung-specific C-type lectin involved in innate immunity.
• SP-D shares structural similarities with SP-A and other collectins, which bind to C1q receptors on macrophages.
• The specific receptor for SP-D on alveolar macrophages and its relationship to SP-A and C1q receptors remain unclear.

Purpose of the Study:

• To investigate the specific binding of SP-D to alveolar macrophages.
• To determine if SP-D utilizes the same receptor as C1q on alveolar macrophages.
• To characterize the binding kinetics of SP-D to alveolar macrophages.

Main Methods:

• Radiolabeling of human SP-D with 125I.
• Isolation of alveolar macrophages from human bronchioalveolar lavage fluid and bovine lung washings.
• Binding assays performed in the presence of EDTA to chelate calcium ions and at 4°C to prevent internalization.

Main Results:

• 125I-SP-D demonstrated specific, time- and concentration-saturable binding to alveolar macrophages.
• Binding was inhibited by approximately 90% with excess unlabeled SP-D.
• The apparent dissociation constant (Kd) for SP-D binding was determined to be (3.6 ± 1.3) x 10⁻¹¹ M.
• C1q also bound to alveolar macrophages (Kd 3 x 10⁻⁶ M), but did not inhibit SP-D binding.

Conclusions:

• SP-D binds specifically to alveolar macrophages.
• The receptor for SP-D on alveolar macrophages is distinct from the C1q receptor.
• These findings elucidate a specific interaction mechanism between SP-D and macrophages in the lung.